It is a common practice to employ filtration systems in connection with metal machining operations using liquid coolants and lubricants. As the liquid carries away metal chips, turnings, and other material from the work areas, it must be clarified before being again pumped through the system or discharged to the sewer. A conventional form of apparatus used in such filtering processes comprises a settling tank and a clean-liquid tank with a filter screen separating the tanks and through which the liquid to be clarified is caused to pass. The filtration efficiency increases as small contaminants build up on the screen to form a bed on the surface thereof, since these contaminants also serve to block the flow of other solid materials in the liquid. However, as the efficiency increases with the progressive clogging of the filter, the rate of flow of liquid from the settling tank to the clean-liquid tank decreases and, if permitted to progressively decrease, would eventually approach zero.
It is common practice to provide a flight conveyor having transverse flights which scrape the bottom of the settling tank and remove the sludge through a discharge chute. Heretofore apparatus has been constructed wherein a return run of the flight conveyor is used to scrape the bed or cake forming on the surface of the filter to thereby prevent an undue build-up. Such a clarifying apparatus has been in use for several years at the Oldsmobile Division of General Motors Corporation in Lansing, Mich. Also, in U.S. Pat. No. 3,748,835 there is disclosed a filter having a downwardly opening face which is scraped by the return run of a flight conveyor.
One of the difficulties with these prior art arrangements is that it is necessary to index the flight conveyor if there is a large chip or swarf load in the settling tank and this in turn will cause the return run of the flight conveyor to scrape the filter cake on the septum even though it may not be desirable to scrape the cake. Another disadvantage of such prior art arrangements is that the septum or filter screen is not readily accessible for cleaning or replacement. In addition, if there is an unusually large chip or swarf load on the filter, it may cause the flight conveyor to jam, requiring an expensive and time consuming shut-down of the clarifier. Finally, the aforementioned prior art operated on a pressure differential across the filter septum of from 0 to 3 psi and this required a relatively large area filter to obtain the flow volumes required for many applications and effectively limited the septum slots or openings to not less than about 0.002 inches.